Since plants can not escape from applied pathogens because of their immobility, they must be able to defend themselves against the pathogenic challenge by direct or indirect response; and, most plants appear to undertake some general defense mechanism to protect themselves from the applied pathogens, e.g., fungi, bacteria and virus.
In this connection, crude extract isolated from Phytolacca americana L. has been reported to inhibit in vivo polypeptide synthesis(see: Owens, R. A. et al., Virology, 56:390-393(1973)); and, said report has accelerated studies on the phytolacca antiviral protein("PAP") isolated from Phytolacca americana L. Under these circumstances, PAPs such as PAP-I and PAP-II produced in spring and summer, respectively, and PAP-S produced from seed, have been discovered and isolated since the early 1970's(see: Irvin, J. D. et al., Arch. Biochem. Biophys., 169:522-528(1975); Irvin, J. D. et al., Arch. Biochem. Biophys., 200:418-425(1980); Barbieri, I. et al., Biochem. J., 203:55-59(1982)).
On the other hand, as a result of extensive studies on the PAP at the molecular level, it was determined that PAPs inactivate the 60S ribosomal subunit of eucaryotic polypeptide synthesis machinery, which is a general phenomenon as other ribosome-inactivating proteins(RIPs) inactivate said subunit (see: Houston, L. L. et al., J. Biol. Chem., 258:9601-9604 (1983)). Further, it has been reported that: PAP is synthesized and secreted from the cytosol and involved in the control of pathogenic virus; however, the detailed mechanism of virus inactivation has not been proved(see: Ready, M. P. et al., Proc. Natl. Acad. Sci., USA, 83:5053-5056(1986)).
Recently, structure and base sequence of PAP genome, a multigene family, have also been characterized and determined (see: Lin, Q. et al., Plant Mol. Biol., 17:609-614(1991)); and, therefore, efforts have continued to exist in the art, for the development of expression vectors for PAP and transgenic plants transformed therewith.